The goal of the proposed experiments is to understand, at a molecular level the role of the target muscle in promoting differentiation of the presynaptic neuron. Motor neurons in culture with muscle grow and contact the muscle, synthesize synapse-specific proteins, and concentrate these proteins at sites of nerve-muscle contact, mimicking processes that occur during embryonic development. A quantitative assay for the synaptic vesicl protein p65 will be developed, and used to test effects of muscle on the regulation of specific protein synthesis. Second, specific antibodies to cell adhesion molecules and extracellular matrix protein receptors will be used to test the role of these proteins in triggering synaptic differentiation. Third, monoclonal antibodies will be raised that recogniz embryonic or denervated but not adult innervated muscle, to find novel molecules that may be involved in synaptogenesis. Finally, the intracellular events involved in presynaptic differentiation will be examined by focusing on perturbations in common intracellular "second messengers." An understanding of the molecules and processes involved in the normal differentiation of synapses is important in order to understand how these processes go awry in disease states. Besides being directly valuable for understanding motor neuron trophic disorders (such as amyotrophic lateral sclerosis) and neuromuscular diseases (such as Eaton- Lambert syndrome), the information obtained will bear in general on developmental disorders of the brain and spinal cord.